This invention relates generally to magnetic resonance imaging (MRI) and more particularly to MRI using ligands as contrast agents.
For several years, magnetic resonance imaging (MRI) has been proposed as a valuable aid in diagnosing and locating malignant tumors. It has, for example, been known that certain paramagnetic metal ions localize in malignant tumors and decrease the relaxation time of hydrogen ions present in water there. Nevertheless, the use of metal ions as contrast agents has been hampered by their inherent high toxicity.
Recently, it has been found that porphyrin and many of its derivatives also localize in malignant tumors. Unfortunately, free porphyrins do not sufficiently increase the relaxation time of water to be effective MRI contrast agents.
Metalloporphyrins have, to some extent, been used to achieve higher contrast on X-rays of neoplastic tissue. One difficulty observed with the use of metalloporphyrins in this context has been the tendency of some metalloporphyrins to dissociate and release the highly toxic metal ion. Additionally, it is noted that the characteristics of a suitable X-ray image enhancer and a suitable magnetic resonance image enhancer are quite different.
It has, however, been proposed that manganese III meso-tetra(4-sulfonatophenyl)porphine (TPPS.sub.4) could be used as a magnetic resonance image enhancer for the deletion of tumors. While such a compound had certain advantages over previous enhancers, the toxicity, tumor uptake, tumor localization and relaxation time were still not as good as desired.